1. The Field of the Invention
The present invention relates to a new class of hydroborate compounds, and to the use of these compounds as reducing agents. In particular, the present invention relates to a family of organocyanohydroborates which may be used as chemically selective reducing agents in a variety of important chemical reactions.
2. The Prior Art
Hydroborate compounds are known to be useful as reducing agents in a variety of important chemical reactions and are important compounds in modern organic synthesis. Additionally, hydroborates may be coordinated to a metal so as to provide certain reducing and catalytic properties. However, the applications of known hydroborate compounds are unfortunately often limited because of the scope of their reducing properties.
Examples of hydroborate compounds used in the prior art include: (1) trialkylhydroborates, (2) cyanotrihydroborates, (3) acetoxyhydroborates, (4) carboxylatotrihydroborates, and (5) monoalkyltrihydroborates. The trialkylhydroborates include trisubstituted hydroborate anions of the following general formula: R.sub.3 BH.sup.-. The trialkylhydroborates are generally considered strong reducing agents.
The cyanotrihydroborate compounds include an anion having the general formula: BH.sub.3 CN.sup.-. In contrast to the trialkylhydroborates, the cyanotrihydroborate compounds are generally considered to be mild reducing agents. Thus, there is a relatively large gap between the reducing potentials of the cyanotrihydroborates and the trialkylhydroborates.
In connection with the milder reducing potential, the addition of the cyano (CN) moiety directly to the boron in the cyanotrihydroborate compounds has resulted in much more useful reagents for the selective reduction of some organic functional groups, particularly in view of its stability at low pH. The electron-withdrawing effect of the cyano group is thought to be responsible for the broader applicability of the cyanotrihydroborate compounds at low pH. Thus, the cyanotrihydroborate compounds have provided for some reducing versatility not experienced with other hydroborate reducing compounds.
Indeed, cyanotrihydroborates have found utility in the following exemplary areas: trapping carbonium ions, direct synthesis of NaBD.sub.3 CN and NaBH.sub.3 CN-t, curing a liquid nitrile polymer and a polymer made from an aliphatic mercaptan and a conjugated diene, reductive bleaching of groundwood pulp, sulfate pulp, and chemi-ground wood pulp without corrosion of the equipment, fluorescent labeling of saccharides, the reductive amination of proteins, synthetic studies on glycocinnamoylspermidines, studies producing alpha-2 antagonism, the reduction of rhodopsin, the regioselective reduction of phenylthiazines, the reductive methylation of proteins, the study of the synthesis and dopamine receptor binding of exo- and endo-2-amine-6,7-dihydroxybenzonorbornene, and the reductive ring opening of carbohydrate benzylidene acetals.
The acetoxyhydroborates contain an anion having the general formula: H.sub.3-x B(CO.sub.2 CCR.sub.3).sub.x.sup.- ; the carboxylatotrihydroborates incorporate an anion having the general formula: H.sub.3 BCO.sub.2.sup.2- or H.sub.3 BCO.sub.2 R.sup.- ; and the monoalkyltrihydroborates contain an anion having the general formula: RBH.sub.3.sup.-. Unfortunately, each of these anions is typically difficult to make and difficult to isolate in a pure form. Thus, the prior art acetoxyhydroborates, carboxylatotrihydroborates, and monoalkyltrihydroborates, have not provided practical reducing agents because of the difficulty in manufacturing and purifying these hydroborate compounds.
In view of the advantages and diversity exhibited by the prior art cyanotrihydroborate compounds, it would be a significant advancement in the art to further diversify the reducing capabilities of such cyanotrihydroborate compounds so as to span the gap of reducing potential between the prior art cyanotrihydroborates and trialkylhydroborates. Additionally, it would be an advancement to provide cyanotrihydroborate compounds which exhibit a greater degree of control over the extent and selectivity of the chemical reduction accomplished.
From the foregoing, it will be appreciated that what is needed in the art are hydroborate compounds which have a wider versatility of reducing capability and which have a reducing capability between the strong and mild reducing hydroborate compounds presently known in the art. It would be another significant advancement in the art to provide a class of hydroborate compounds which are relatively easy to make and to isolate in pure form.
Moreover, it would be another significant advancement in the art to provide a new class of hydroborate compounds which incorporate a cyano moiety and other substituent groups which provide for a class of hydroborate compounds having not only all of the advantages of the prior art cyanotrihydroborate compounds, but also greater versatility and wider application as reducing agents.
Such hydroborate compounds and methods for manufacturing and using such compounds as reducing agents are disclosed and claimed herein.